The present invention relates to a hydraulic power steering apparatus in which the steering assist power can be varied by controlling a stepping motor according to a vehicle driving condition.
In order to vary the steering assist power according to the vehicle driving condition, for example, vehicle speed in a hydraulic power steering apparatus, a stepping motor that moves a spool of a variable throttle valve provided in a hydraulic circuit is driven according to the number of steps that is set according to the vehicle driving condition (see Patent Document 1).
In order to set accurately the degree of opening of a variable throttle portion in such variable throttle valve, it is necessary to perform accurate positioning of the spool by controlling the stepping motor. For this purpose, the origin corresponding to the actual reference position of the spool is required to match the origin for calculation that serves as a reference for calculating the number of steps.
Accordingly, a stopper that can stop the movement of the spool in one direction is provided in the origin, the number of steps for setting origin that is required for moving the spool to the origin is predetermined, and the stepping motor is driven according to the number of steps for setting origin at the time when the control is started. As a result, the spool is positioned in the origin at the time when the driving of the stepping motor according to the number of steps for setting origin is completed therefore, the value of the number of steps at this time is set as the origin for calculation.    Patent Document 1: Japanese Patent Application Laid-open No. 2000-229578